Existing schemes for achieving EMI suppression are numerous. Nevertheless, the art has had little or no success in developing a cost-effective EMI filter that mounts directly onto a printed circuit (PC) board and that accommodates a large number of PC board-defined conductors (PC conductors).
An example of an existing EMI filter is a large ferrite core, which is a large (up to 5 cm or more in diameter) cylinder of ferrite material through which a bundle of signal-conducting wires is looped. Because of its size, the ferrite core is expensive and cannot be mounted directly onto PC boards. Moreover, because the conductors must be looped through the center of the core, the core cannot directly accommodate PC conductors, which are printed on the PC board. Since the large ferrite core is an off-board EMI suppression solution, its performance is not optimized because its physical distance from the source causing the EMI interference is not short. Another example of an existing EMI filter is a capacitive connector, which is a connector that has each pin or socket connected to ground by a shunting capacitor. Due to the large number of components involved, a capacitive connector is not cost-effective. Moreover, the capacitors cause significant attenuation not only of the interfering signals, but of the desired signals as well, and they are effective only at high frequencies (greater than about 200 MHz). Another drawback is that capacitive connectors are not transparent to differential (bidirectional) mode signals, such as tip and ring phone signals. Although capacitive connectors are PC board-mountable, they are limited in the number of PC conductors that they can accommodate--typically fewer than 50. Yet other examples of existing PC board-mountable EMI filters are in-line ferrite devices such as those shown in FIGS. 1-6. Due to its geometrical structure, the filter of FIGS. 1-2 provides low, and therefore often inadequate, EMI suppression. The filter of FIGS. 3-4 also provides low suppression, plus it has a low capacity in the number of signal lines that it can reasonably accommodate. Also, this device is not transparent to differential (bidirectional) signals. Finally, the filter of FIGS. 5-6 also has a low capacity in the number of signal lines that it can accommodate, because its performance degrades rapidly with increases in its size (its circumference). This device is also not transparent to differential signals. Furthermore, just like the ferrite core, it cannot directly accommodate PC conductors.